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THE TRANSPORTATION OF GASES
147
of air. The lines slightly inclined to the vertical represent the absolute pressure in pounds per square inch. The lines at an angle of 45 deg. represent temperature in degrees Fahrenheit and intrinsic (internal) energy in B.t.u., and the dotted inclined lines represent entropy. These last lines are paralleled in finding the results of an adiabatic change, while if the change follows a piVin — p^V^1 path, the effects are studied by paralleling lines indicated at the margin for various values of n. For example, an inspection of Fig. 1 shows that 1 Ib. weight of air at a pressure of 125 Ib. absolute per square inch and occupying a volume of 1.5 cu. ft. will have a temperature of 50°F. and contain 85.86 B.t.u. or 66,800 ft.-lb. of internal energy.
Furthermore, the adiabatic expansion of 1 Ib. of air from a pressure of 125 Ib. absolute per square inch and a temperature of 70°F. to a pressure of 14.7 Ib. absolute
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FIG. 1.—Diagram for determining the amount of energy stored in compressed air.
will result in a final temperature of — 170°F. and the work done during the expansion would be (89.22 - 48.79 =) 40.43 B.t.u. or 31,454 ft.-lb. per pound of air. This expansion follows the dotted line of the diagram, but if the expansion followed the equation piFi1-2 = p^Vz1'2, the available intrinsic energy per pound for any pressure range would be found by following a line parallel to the marginal line of the chart marked n = 1.20 from the upper to the lower pressure. E.g., if such an expansion occurred between 125 Ib. absolute per square inch and 70°F. and 14.7 Ib. absolute, the resulting temperature would be — 87°F. and the work done would be (89.22 - 62.76 =) 27.46 B.t.u. or 21,346 ft.-lb. per pound of air.
With adiabatic compression, the final temperature 772 (degrees Fanrenheit absolute) is given by the formula Tz = Ti(p2/pi)°^ for single-stage compression; by